1. Field of the Invention
The present invention relates to a method for forming an extended gate field effect transistor (EGFET) based sensor, and in particular relates to a method for forming titanium dioxide, ruthenium doped titanium dioxide and ruthenium oxide sensing films.
2. Description of the Related Art
Titanium dioxide (TiO2) material has advantages such as a high refractive index, a high dielectric constant, stable chemical properties, well insulation, high hardness, and wear-resistance, etc., and thus is usually applied in high refractive index films, gas sensor films and wet sensor films of optical interference multi-layer film filters, anti-reflection films and high reflection films, and microelectronics devices.
Ion sensitive field effect transistor (ISFET) is a product from the electrochemical field that is combined with semiconductor technology and has the advantage of scaling and is suitable for automatic measurement. ISFET has ion sensitive functions and field effect transistor properties. Therefore, ISFETs are different from traditional ion-selective electrodes (ISE). In comparison with traditional glass electrodes, an ISFET has advantages of cable of scaling and measuring trace amounts of a solution, high input impedance, low output impedance, being a good generalized impedance converter, fast response time, process compatibility with a MOSFET, manufacturing ease, and being applied in development of biosensors, etc.
Extended gate field effect transistors (EGFET) are developed from ISFET devices and the cost thereof is low and structure thereof is simple. EGFETs can solve the packaging problem of ISFETs. EGFETs stabilize the metal gate when MOSFETs are being formed and separate the sensing film from the gate of the field effect transistor, wherein the sensing film and the gate of the field effect transistor are connected by a signal wire. The binding behaviors of the ion sensing films of ISFETs and EGFETs are the same and the difference between ISFETs and EGFETs is that ISFETs need an insulation film having high resistivity and high dielectric constant while EGFETs transfer the interfacial potential formed from the ion sensing film by a signal wire, and thus a conductive film with low resistivity is needed to transfer the potential.
Traditional ISFETs and EGFETs are only used to measure the hydrogen ion concentration of a solution. If a sensing film is coated with a specific enzyme, the enzyme film will interact with a specific substrate in the solution and result in hydrogen ions changing the interfacial potential of the sensing film. Then, the changed hydrogen ion concentration of the solution is measured by and ISFET and EGFET to detect the concentration of the specific substrate in the solution. The ISFET and EGFET mentioned above are called enzyme field effect transistors (ENFETs).
Ion-selective electrodes and biosensors are combined with molecular biology and electronic technologies, to develop electrical instruments and measurement products related to medical treatments. According to the definition from the National Academies, biosensors are integrated extensive products of molecular activity or active systems, which are applied to sensing devices and measurement systems such as enzyme molecules or protein antibodies. The measuring instruments are provided to detect relative specific substances in the environment. Bio-chip measurement systems belong to one of the biosensor system, which not only monitors and detects organisms and physiological parameters for real time or a long time study, but also is applied to detect contents of foods and the environment.
U.S. Pat. No. 4,877,582 discloses a chemical sensor having a field-effect transistor as an electronic transducer that is used for the analysis of specific constituents in a liquid. The chemical sensor comprises means for which an externally supplied sample solution is permitted to reach a chemical receptor of the chemical sensor while substantially preventing external light from reaching the field effect transistor. U.S. Pat. No. 6,218,208 discloses a sensitive material-tin oxide (SnO2) obtained by thermal evaporation or by r.f. Reactive sputtering process is used as a high-pH-sensitive material for a Multi-Structure Ion Sensitive Field Effect Transistor. The multi-structure of this Ion Sensitive Field Effect Transistor (ISFET) includes a SnO2/SiO2 gate ISFET or SnO2/Si3N4/SiO2 gate ISFET respectively, which has high performances such as a linear pH sensitivity of approximately 56-58 mV/pH in a concentration range between pH 2 and pH 10. Meanwhile, a low drift characteristic is approximately 5 mV/day and response time is less than 0.1 second. U.S. Pat. No. 6,251,246 discloses a polymeric material, wherein a stable and reproducible interface is between ionic and electronic domains of an ion selective sensor, or an ion selective field effect transistor, or the like is provided. When employed in an ion selective sensor, the polymeric material is advantageously provided over a solid internal reference electrode and an ion selective material is provided thereover. U.S. Pat. No. 4,992,382 discloses a method for measuring calcium ions, wherein a calcium sensitive reagent, calcichrome, is immobilized on a porous polymer film. The reaction of the calcium sensitive reagent to the Ca2+ is then measured and concentration determined as a function of the reaction.
U.S. Pat. No. 4,946,574 discloses an apparatus for the production of sterilized calcium-ion water, which includes a housing structure and an electrolytic cell with electrical terminals and electrodes installed in the housing structure. A plurality of electromagnetic valves separately provided at a water-intake pipe and a water-drain pipe communicates with the electrolytic cell. A setting switch is disposed in the housing structure and is electrically coupled with the electrolytic cell for adjusting the current intensity of electrolysis therewith. A hydraulic-pressure switch is provided at a water pressure pipe for the control of the water-level in the electrolytic cell. Moreover, an ultraviolet sterilizing tank is connected to the electrolytic cell for sterilizing the electrolyzed water. Meanwhile, a control circuit is used to respectively couple with the electrolytic cell, the electromagnetic valves and the setting and hydraulic switches, whereby, calcium-ion water can be effectively produced for drinking purposes. US Patent Publication No. 20060008915 A1 discloses a method for determination of calcium in a sample derived from a living body, by the reaction of calcium in the sample with Chlorophosphonazo-III or a compound analogous thereto in the presence of vanadate ions. Calcium in the sample is determined on the basis of an optical change caused by the reaction product. Following are advantageous of the disclosed. The disclosure eliminates the problem of absorption of carbonic acid gas caused by a high pH and the problem of using a toxic reagent containing arsenic. Additionally, the disclosure makes it possible to test a large number of samples in short period of time due to use of an autoanalyzer. Furthermore, calcium determination of a wide range is permitted because of low sample blank values of the disclosure.
U.S. Pat. No. 5,102,527 discloses a calcium ion selective electrode having a sensitive membrane comprising an organic polymeric substance, a plasticizer and a calcium ion sensitive substance of a neutral carrier type, wherein the dielectric constant of the plasticizer is 10 or less, thus remarkably improving the responsiveness and stability of the electrode. U.S. Pat. No. 5,496,522 discloses a chemical sensor and biosensor probes for measuring low concentration of metals and metal ions in complex samples such as biological fluids, living cells, and environmental samples. The disclosure relates to a gel-based Indo-1 and Fura-2 chemical sensor probes for the measurement of low concentrations of calcium, cadmium, magnesium and the like.